Cable actuators including a screw/nut assembly the screw of which is driven in rotation by an electric motor and the nut of which is mobile in translation are known. The mobile element is coupled to one or more cables to exert traction thereon.
There is known from the document FR2809464 a cable actuator of this kind in which the element mobile in translation is the screw while the nut is driven in rotation by a motor. The cable enters a hole in the screw and is coupled by means of an attachment tolerating misalignments of the cable.
Such a cable actuator may be used to actuate a robot arm segment or finger bone. In these joints, the mobile element is rigidly connected to a rotary shaft that includes a pulley onto which the cable is wound. To increase the movement of such a joint beyond one rotation of the shaft, it is necessary to allow winding of the cable on itself, which has the disadvantage of increasing the primitive radius of the winding. Using a pulley with a helical groove whereby the primitive winding radius remains constant may be envisaged, but such a pulley causes progressive misalignment of the cable as the cable is wound, which generates transverse forces that interfere with the operation of the screw/nut assembly.
Moreover, in some applications, notably in robotic applications, the overall size of the actuator is highly critical and it is important to make this overall size as small as possible.
In this regard cable actuators are known that include a screw rotatably mounted and driven by an electric motor, a nut cooperating with the screw and associated with anti-rotation means such that rotation of the screw by the motor causes axial movement of the nut, and two substantially parallel cables coupled to the nut on either side thereof.
For the same travel, this device makes it possible to reduce the overall size of the cable actuator. In fact, in known cable actuators in which the or each cable is coupled to the screw that moves, it is necessary to allow the screw to project on either side of the actuator. The general overall size would therefore be at least 2C+L, where C is the stroke of the actuator and L the overall size of the nut. In the actuator in which the nut moves, this minimum overall size is only C+L.